Process of calendering mineral-coated paper and product



Patented July 2, 1929.

UNITED STATES 1,719,166 PATENT OFFICE.

DONALD B. BRADNER, 0F HAMILTON, 01110, ASSIGNOR TO THE CHAMPION COATED PAPER COMPANY, OF HAMILTON, OHIO,

A CORPORATION OF OHIO.

PROCESS OF CALENDERING MINERAL-COATED PAPER AND PRODUCT,

No Drawing.

This invention relates to a new method tor developing finish on mineral coated paper, and to the mineral coated paper produced thereby.

By the term Mineral coated paper I mean to designate paper having a top coating whose composition on the dry basis comprises at least 70% finely divided mineral filling matter and not more than 30% adhesive. Mineral coated paper as thus defined is commonly used for book and magazine printing purposes.

In making mineral coated paper it is customary first to apply to the body stock a coating mixture containing a pigment and a binding agent suspended in water. The coat.- ing mixture is applied in a variety of ways, such as for example, by means of brushes, rolls or sprays. To obtain mineral coated paper of good quality it is necessary to exercise considerable care in applying the coating mixture in order to secure as even a coating as is ossible. The coating n'iixture itself must "e sutiiciently fluid to avoid brush marks. streaki'ngs and other imperfections in the finished paper. Various devices have been used to smooth the wet coating, but in drying by the usual method of contacting with air the coating loses much oi the smoothlless so developed and the resulting paper when dry and before calendering is veryrough as observed with a hand lens and has no gloss or finish.

After the coating mixture has been applied and dried the mineral coated paper is calen dered by passing it between the rolls of a super calender. which is essentially a stack of alternate steel and either cotton or paper rolls.

In another method of calendering especiallv adapted to card board the substantially dry sheets are laid between zinc plates and passed back and forth under a steel roll.

The degree of gloss or finish produced in the ordinary calendering operation depends on a number of factors, such as for example, the pressure applied to the rolls, the amount of moisture in the paper thephysieal condition of the surface of the rolls, the temperature of the rolls, the speed of operation and the nature of the coating mixture. some of these factors are Very difficult to control in actual practice, with the result that the calendered paper is likely to be non-uniform. Among thenndesirable features of the usual Application filed January 3, 1927. Serial No. 158,887.

ealendering process may be mentioned the high power consun'iption. the excessive amount of time required to keep the rolls in condition, the danger to the operators and the fact that it is a separate operation from the coating process and requires an extra handling of the paper rolls. Also when attempts are made to produce high finish-by using high pressures on the super calender the paper is frequently blackened and weakened.

Mineral coated paper having a high tinish is generally more valuable than paper having a low finish. Various methods have been employed to secure highly finished paper. Thus satin white has been used in coating mixtures largely for this purpose. Varnisheshavealsobeen applied for the same purpose. tubbing down by hand and polishing with brusheshave also been employed to develop high finished paper. All of these methods are relatively expensive.

One object of the present invention is to provideamethodot obtaining paperhaving an extremely high finish. Furthermore, the invention provides a method of obtaining paper having practically any degree of fin sh desired. A further object is to provide a simple, less dangerous, less expensive and more easily controlled method of producing mineral coated paper.

The essential feature of my invention is the pressing of the mineral coated paper against a solid surface while the coating is in ahighly plastic condition and rendering the coating comparativelynon-plastic before it is removed from this surface. For e);-v ample, it the coating is plastic when wet it is dried in contact with the solid surface; or if the coating is plastic while hot it is cooled in contact with the solid surface. In this way the coating takes on the finish of the surt'ace with which it is in contact. Thus in the case of a coating which is plastic when wet, it the paper dries with the coating in contact with a very smooth, highly polished surface, the paper will have a very high finish. On theother hand it' the paper dries with the coating in contact with a smooth but unpolished surface the paper will have a correspondingly lower finish. Similar results may be obtained with coatings which are plastic while hot and set to a relatively non-plastic state upon cooling. In this case cooling to below the plastic temperature iii) while in contact with a highly )olished surface will produce a very high nish, whereas cooling in contact with a smooth but unpolished surface will produce paper having a lower finish. Practically any degree of gloss or finish, ranging from an extremely high finish to what is known as a dull finish. can be imparted to the coating b a suitable choice of the surface on which the paper is pressed as the coating sets or hardens.

The method of finishing mineral coated paper herein described is also adaptable to producing various markings or designs on the mineral coated paper by contacting the coating while plastic with a solid surface having the desired-markings or designs cut, etched or otherwise inscribed on it.

I have discovered that paper having a high finish may be obtained in this manner without the use of satin white or other expensive agents. In fact a coating containing clay as the only pigment will give a paper by this process having a higher finish than coatings prepared in the usual manner, containing large percentages of satin white.

In carrying out this process it is necessary that the coated side of the paper be contacted with the surface on which it is to harden while the coating is in a plastic condition. One convenient method of accomplishing this is to contact the coated side of the paper with the solid surface by means of one or more rolls pressing down on the opposite side of the paper. This operation has the effect of smoothing out irregularities in the distribution of the coating and eliminating brush marks and similar imperfections. It is furthermore advisable that the coated side be kept in contact with the solid surface throughout the greater part of the hardening operation. This may be accomplished in several ways. Thus the coating and the solid surface may be of such a nature that the paper will stick to the solid surface of its own accord until set and then break loose spontaneously. Or the paper may be kept more or less tightly in contact with the solid surface by external means such as a belt.

Any convenient form of equipment may be used to provide the surface in contact with which the coating is to set. For instance, a moving belt having any desired degree of finish may be used for this purpose. In this case the'paper, freshly coated may be run on to the travelling belt, with the freshly coated 3 side of the paper in contact with the belt.

As the paper passes to the belt it is firmly pressed against the belt by means of one or more rolls. The coating is allowed to set in contact with the belt. After setting the paper may be detached from the belt and wound on a reel in the usual manner. The belt may be made of any suitable material, for example, metal or hard rubber depend- I ing-en the composition of the coating. The

contact or finishifig surface may also take the form of a, revolving drum. In case the coat-ing is one which is plastic while moist, the drum should be steam heated to hasten the drying. In case the coating is one which is plastic while hot, the drum may be cooled with water. In any case I prefer to press the side of the paper to which the plastic coating has been ap )lied against the contact surface by means 0 rolls or other pressure applying means. Continuous contact with the drum may be obtained by the use of a travelling belt made of felt or other suitable material.

Although it is advisable to keep the coated surface of the paper in contact with the finishing surface throughout the greater part of the hardening period, it is not essential that this be the case. For example, the coating may be partially set between the point of application of the coating to the sheet and the point of contact with the finishing surface. Another way of expressing the same idea is that the means for applying the coating does not have to be at any fixed distance from the finishing surface and neither does the paper have to travel at any particular speed. Finally the method does not necessarily require that the hardening be complete in anycase.

It is, of course, necessary for the success of my invention that the coating'shall not stick to the drying surface after the coating has become substantially non-plastic. The coating contains an adhesive and should this adhesive prevent the free release of the coated paper after it has set the coating would be imperfect. It is immaterial whether or not the plastic coating adheres at first to the finishing surface, but it is essential that the coated paper when the coating has become substantially non-plastic be practically nonadherent to the finishing surface.

The selection of a suitable material for the finishing surface will as stated above depend upon the composition of the coating. I have found, for example with starch or casein adhesives, that several materials are satisfactory for this purpose, among which may be mentioned chromium, Monel metal, hard rubher and to a lesser extent nickel and brass. Many other substances to which coatings containing starch or casein ordinarily adhere when dried in the manner herein described, may be rendered non-adherent by treatment with a wax. As examples, I mention copper and steel. In case fusible gums or resins are used as adhesives, chromium is quite satisfactory. The non-adhering surface may be in the form of a plating or coating on a cheaper base.

The method of calendering herein described may in general be used with the coating mixtures commonly employed in the manufacture of mineral coated paper. These coating mixtures generally consist essentially of an aqueous suspension of pigment (usually clay, satin white. etc.) and an adhesive or binding agent (usually casein, glue or starch). Paper havin an extremely high finish may be prepared by the calendering process herein described, using clay and binding a ent alone, but the color and brightness may be improved by the addition of satin white, blanc fixe, etc. Also coating mixtures containing dyes and other minor ingredients often present in the usual coating mixtures, may be used with my method of calendering. This process of calendering is adaptable for heavy or light coatings. Also each side of the paper may be coated successively.

I give the following example of a coating mixture which when applied to a sheet of paper and calendered on a highly polished chromium surface in the manner herein described gives a mineral coated paper of very high quality: 200 pounds clay ((35% solids) and 15 gallons of casein solution (made by dissolving casein in ammonia water and corn t aining about 1.5 pounds casein per gallon) made up with water to a total volume of 30 gallons.

The method of calendering herein de scribed is of particular advantage with coatings containing starch as an adhesive. For example a coating composition containing parts of clay, 20 parts of uncooked corn starch, 120 parts of water and part gum arabic, the parts being by weight, applied to the paper and gelatinized by contact with steam as described in my application Ser. No. 147,200, filed November 8, 1926, may be employed. In preparing mineral coated paper by the usual methods casein is preferred to starch for several reasons among which may be mentioned: (1) the superior finish obtained with casein. and (2) with starch it is more difficult to apply smooth coatings free from brush marks. On the other hand casein is much more expensive and gives printing results inferior to starch. ll'hen the herein described method of calendering is used in connection with an aqueous coating mixture comprising clay as the pig ment and starch as the binding agent. mineral coated paper may be obtained having a higher finish than that obtainable by the use of the more expensive ingredients. casein and satin white, calendered in the usual manner.

The method of calendering herein described is not limited to use with coating mixtures con aining water. The coating mixture may contain a non-aqueous solvent or suspending liquid for the pigment or coating agent, which is evaporated while in contact with the finishing surface. Also, in case the binding agent is plastic when hot, it is not essential that there be any solvent or suspending liquid present.

It will be apparent that the method of calendcring herein described makes it unnecessary to apply the coating in a highly uniform manner since brush marks and other similar imperfections are largely eliminated by the rolls or other pressure applying means used to contact the coating to the finishing surface.

The method is applicable for coating very thin paper, medium weight paper, and also very heavy paper or card board. Card board cannot be successfully calendered on the ordinary calender or super-calender due to the relatively high curvatures of the rolls, which would'cause the card board to crack. If a large drum is employed to simultaneously dry and calender the web in the manner described herein, the angle of curvature is comparatively low with the result that cracking of the card board does not occur.

It will be apparent that mineral coated paper produced by my new process will have a surface corresponding to the surface in contact with which the coating has been allowed to harden. The surface thus produced may be aptly characterized as a cast surface and the term cast surface as used in the following claims is intended to define the character of surface obtained by contacting a mineral coated paper while the coating is in a plastic condition with a solid non-adhering surface having a finish substantially the same as that desired on the mineral coated paper and rendering the coating substantially non-plastic before removing it from the solid surface. Mineral coated paper made according to the process described herein, in which the coating hardcus in contact with a highly polished surface, is characterized by an extremely smooth surface. lfhe surface of this paper is much smoother than even the best grades of mineral coated paper made in the past.

Also, this new paper has an entirely different appearance from the customary mineral coated paper. @rdinary mine 'al coated paper. even the highest grade of twice-coated, high finished paper, when viewed at approximately the angle of the incident light, has a more or less mottled appearance due to the fact that the surface of the coating tends to conform to the irregular surface of the paper base. The new product. on the other hand, whether of the high finish type or of the dull or matt-tinish type, is substantially free from this lack of uniformity. The high-finish product has the smooth, homogeneous appearance of a fine porcelain. The glaze or finish may be extremely high, resembling or surpassing a high grade varnished, coated paper in this respect. However, in the new product, it is the coating itself which provides this surface and finish, and not a film of varnish or other costly, more or less colored, diliicultly printable material.

In the following claims the term non-adits herin'g surface means a surface from which the mineral coated aper can be easily removed aftcr the coating has set, without. destroyin the finish imparted to the coating by condition with a solid non-adhering surface having a-ifinish substantially the same as that desired on the mineral coated paper, rendering the coating substantially non-plastic, and

thereafter removing the mineral coated paper from the solid surface. 2. Process of finishing mineral coated paper which comprises contacting the coated side thereof while the coating is in a plastic condition with a highly polished non-adhering surface, rendering thecoating substantially non-plastic, and thereafter removing the mineral coated paper from the surface.

3. Process of finishing mineral coated paper which comprises, contacting the coated side thereof while the coating is in a plastic condition with a revolving drum having a non-adhering surface, rendering the coating substantially non-plastic, and thereafter removing the mineral coated paper from the drum. p

41 Process offinishing mineral coated paper which comprises contacting the coated side thereof while the coating is in a plastic condition with a chromium surface, render: in g the coating substantially non-plastic, and thereafter removing the mineral coated paper from the chromium surface.

5. Process of finishing mineral coated paper which comprises drying the coating in contact with a solid non-adhering surface having a finish substantially the same as that desired 0n the mineral coated paper.

' 6. Process of finishing mineral coated paper which comprises drying the coating in contact with a highly polished non-adhering surface.

7. Process .of finishing mineral coated paper which comprises drying the coating in contact with a heated revolving drum having a non-adhering surface.

8. Process of finishing mineral coated paper which comprises drying the coating in contact with a chromium surface.

9. Process of finishing mineral coated paper which comprises drying a coating containing water in contact with a solid non-adhering surface having a finish substantially the same as thatdesired on the mineral coated paper.

' 10. Process of finishing mineral coated paper which comprises drying a coating containing water in contact with a highly polished neg-adhering surface.

11. Process of finishing mineral coated paper which comprises drying a coating containing water in contact with a heated revolving drum having a non-adhering surface.

12. Process of finishing mineral coated paper which comprises drying a coating containing water in contact with a chromium surface.

13. Process of finishing mineral coated paper which comprises drying a coating containing starch as a binding agent in contact with a solid non-adhering surface having a finish. substantially the same as that desired on the mineral coated paper.

14. Process of finishing mineral coated paper which comprises drying a coating containing starch as a binding agent in contact with a highly polished non-adhering surface.

15. Process of finishing mineral coated paper which comprises drying a coating containing starch as a binding agent in contact witha heated revolving drum having a non adhering surface.

16. Process of finishing mineral coated paper which comprises drying a coating containing starch as a binding agent in contact With a chromium'surface.

17. Process of making mineral coated paper which comprises applying a fluid to plastic adhesive mineral coating composition to the paper web or sheet and drying the coating to a substantially non-plastic condition in contact with a non-adhering solid surface.

18. Mineral ,coated paper having a coating con'iprising at least of a mineral filling material and not more than 30% of an adhesive, and having a cast surface.

19. Mineral coated paper having a coating comprising clay and an adhesive, and having a cast surface.

20. 'Mineral coated paper having a coating comprising at least 70% of clay and not more than 30% of an adhesive, and having a cast surface.

21. Mineral coated paper having a coating comprising at least 70% of clay and not more than 30% of starch, and having a cast surface.

In testimony whereof, I aflix my signature.

DONALD B. BRADNER. 

